In order to obtain high sensitivity, reduction sensitization has been investigated for a long time. Disclosed reduction sensitizers include Tin compounds, in U.S. Pat. No. 2,487,850 by Carrol; polyamine compounds, in U.S. Pat. No. 2,512,925 by Lowe et al.; and thiourea dioxide compounds, in GB Patent No. 789,823 by Fallens et al. Furthermore, physical properties of silver nuclei produced by a variety of reduction sensitization methods were compared by Collier, as described in Photographic Science and Engineering, 23, p. 113 (1979). She employed several methods using dimethylamine borane, stannous chloride, hydrazine, high pH ripening, and low pAg ripening, for this purpose. Further, reduction sensitization methods are disclosed in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777, and 3,930,867. A means of reduction sensitization, as well as a selection of reduction sensitizers, is described in JP-B ("JP-B" means examined Japanese patent publication) Nos. 33572/1982 and 1410/1983. Moreover, a method of improving the storage stability of reduction-sensitized emulsions is disclosed in JP-A ("JP-A" means unexamined published Japanese patent application) Nos. 82831/1982 and 178445/1985. Further, Takada et al. disclose a method of preparing an emulsion reduction-sensitized with thiosulfonic acid, in JP-A No. 191938/1990. Though many investigations were tried as mentioned above, these reduction sensitization methods were not adequate for providing sufficiently high sensitivity, as compared to hydrogen sensitization, wherein a photosensitive material is subjected to hydrogen gas processing. This is reported by Moisar et al. in the Journal of Imaging Science, vol. 29, p. 233 (1985).
On the other hand, JP-A No. 162546/1984 discloses that a hydroxyl amine compound improves latent image intensification. However, this method did not give high sensitivity to a silver halide emulsion.